Calculate the number of occurrences of repeated items in the Java List

Package com. qiyadeng. core; import java. util. arrayList; import java. util. collections; import java. util. hashMap; import java. util. hashSet; import java. util. list; import java. util. map; import java. util. set; import java. util. treeMap; public class CountDuplicatedList {public static void main (String [] args) {List list = new ArrayList (); list. add ("a"); list. add ("B"); list. add ("c"); list. add ("d"); list. add ("B "); List. add ("c"); list. add ("a"); list. add ("a"); list. add ("a"); System. out. println ("\ n Example 1-calculate the number of times 'A' appears"); System. out. println ("a:" + Collections. frequency (list, "a"); System. out. println ("\ n Example 2-calculate the number of times all objects appear"); Set uniqueSet = new HashSet (list); for (String temp: uniqueSet) {System. out. println (temp + ":" + Collections. frequency (list, temp);} System. out. println ("\ n Example 3-use Map to calculate the number of times an object appears"); Map map = New HashMap (); for (String temp: list) {Integer count = map. get (temp); map. put (temp, (count = null )? 1: count + 1);} printMap (map); System. out. println ("\ nMap sorting-sort by key"); Map treeMap = new TreeMap (map); printMap (treeMap);} public static void printMap (Map map Map) {for (Map. entry entry: map. entrySet () {System. out. println ("Key-value:" + entry. getKey () + "-" + entry. getValue () ;}} output result Example 1-calculate the number of occurrences of 'A' a: 4 EXAMPLE 2-calculate the number of occurrences of all objects d: 1b: 2c: 2a: 4 example 3-use Map to calculate the number of times an object appears Key-value: d-1Key-value: B-2Key-value: c-2Key-value: a-4Map sort-Sort key-value by Key: a-4Key-value: B-2Key-value: c-2Key-value: d-1